He also recommended people use mosquito repellant, wear long sleeves, repair screens and drain any standing water to prevent the spread of the mosquitoes.

"In Miami, aggressive mosquito control measures don't seem to be working as well as we would have liked," said Frieden.

He said it was possible that mosquitoes are resistant to insecticides currently being used, or that they may have hidden breeding areas that haven't been found yet, or that this type of mosquito - the Aedes aegypti - is simply difficult to control.

Frieden said most people with Zika do not show any symptoms.

"Nothing that we have seen indicates widespread transmission but it is certainly possible there could be sustained transmission in small areas."

Al Jazeera's Andy Gallacher, reporting from Miami, said: "The authorities here are concerned about people's health but there could also be an impact on Florida's tourism. It is the number-one industry here.

"But the overwhelming majority of experts here said they don't expect Zika to spread [in the US] in the same way as we have seen in Brazil or Puerto Rico where the infrastructure isn't quite as robust."

First local cases in US

On Friday, Florida officials announced the first locally transmitted cases of Zika in the US, with all four linked to the same area in Miami.

Early on Monday, Governor Rick Scott said the number of identified cases had jumped by 10 to 14.

The cases mark the first time the Zika virus, which can cause birth defects and is considered particularly dangerous for pregnant women, is known to be spreading via local mosquitoes in the US.

More than 1,600 cases of Zika have been previously reported in the US, but most were brought by travellers who were infected elsewhere. The virus can also spread by sexual contact.

The CDC is sending an emergency team of specialists to help the Florida response, Frieden said.